Highly Selective Formation of [4+2] and [4+3] Cycloadducts of Tetrahydroindenes Generated in Situ from a (1-Alkynyl)carbene Tungsten Complex by the Metalla-1,3,5-hexatriene Route

1-Amino-3-ethoxytetrahydro-3aH-indenes 8 can be readily generated together with pentacarbonyl(pyridine)tungsten in a template induced three-component reaction of [(1-cyclohexenyl)ethynyl]carbene tungsten complex 1 with secondary amines 2a-d and pyridine. Even though the compounds 8 are thermally quite unstable and undergo a fast rearrangement to tetrahydro-7aH-indenes 7. they can be trapped by formation of (rather strained) [4+2] cycloadducts 12 with maleimide 11. If 1-amino-3-ethoxytetrahydro-3aH-indenes 8 are generated in the presence of electron-poor alkynes 2a and 2b, they undergo a 1,5-shift to give tetrahydro-7aH-indenes 7, which in turn afford [4+2] cycloadducts 4a-d. Condensation of 1-tungsta-1,3,5-hexatrienes (3E)-5a-d with 1-metalla-1,3-butadienes 14 (M = Cr, W) give [4+3] cycloadducts 15a-e of tetrahydro-7aH-indenes 7 in good yields with high regio- and stereoselectivity.